The present invention relates to a space diversity communications system for a mutli-direction time division multiplex (MD-TDM) communications network, which consists of a number of scattered satellite stations and one common central station.
In a conventional MD-TDM communications network, time division multiplexed (TDM) signals are simultaneously transmitted to a plurality of satellite stations in a continuous mode. Each satellite station, on the other hand, transmits burst signals to the central station in a time slot assigned thereto and synchronized to a clock at the central station. The burst signals from the respective satellite stations are regularly aligned in a time sequence at the central station so as to be identified and reproduced according to the clock at the central station.
In such an MD-TDM communications sytem, where the transmission path between the central station and a given satellite station extends over a long distance or a sea, the radio transmission quality requires compensation preferably by the use of a space diversity system.
Meanwhile, as a space diversity system in two-way communications between two geographically separated stations, there is available a baseband switching system in which signals received from a main and a supplementary antennas are separately demodulated, and whichever demodulated signal having a lower bit error rate is selected.
The problem encountered with this switching sysem is that, if it is directly applied to the previously mentioned MD-TDM communications sytem, the central station which is receiving signals from a plurality of satellite stations has to monitor the channel conditions and control the radio transmission quality for each satellite station.
Furthermore, where a main and a supplementary antennas are installed in the central station, difficulty is experienced in response to a need for space diversity communication between satellite stations in selecting an optimum distance between the two antennas for the respective satellite stations to which different communications paths are assigned, resulting in a limited space diversity effect.